The in vitro T helper cell (Th) response of human peripheral blood leukocytes (PBL) to HLA alloantigens is mediated by three distinct Th-antigen presenting cell (APC) pathways: 1) CD4+ Th and autologous APC (CD4-sAPC); 2 ) CD4+ Th and allogeneic APC (CD4-aAPC); and 3) CD8+ Th and allogeneic APC (CD8-aAPC). There is a hierarchy of sensitivity of these pathways to the immunosuppressive effects of cyclosporin A (CsA) and FK506, such that the CD4-sAPC is the most sensitive and CD4-aAPC is the least sensitive, when these drugs are added to cultures of PBL from healthy donors. We tested the in vitro Th function of more than 150 human renal allograft recipients on multi-drug immunosuppressive therapy. Our results indicate that only the presence of a functionally intact CD4-sAPC pathway correlated with chronic or acute graft rejection. Our findings suggest that this approach can be used to monitor the graft status of organ transplant recipients.